A Petri dish, also called a Petri plate, is a shallow, cylindrical, flat-bottomed dish with a loose-fitting lid used primarily to culture microorganisms in a laboratory environment. It provides a contained space for solid growth media such as agar, enabling the growth and isolation of bacteria, fungi and other small organisms under controlled conditions.
History and Construction
The Petri dish was introduced in 1887 by German bacteriologist Julius Richard Petri as a refinement of earlier culture methods used by Robert Koch. Modern dishes are usually made from clear borosilicate glass or disposable polystyrene and range from 35 mm to 150 mm in diameter. The dish consists of two parts: a shallow bottom that holds a solid or semi-solid growth medium and a slightly larger lid that protects the contents from airborne contaminants while still allowing gas exchange. Media such as nutrient agar, blood agar or selective formulations are poured into the base and allowed to solidify. Before inoculation, the dish and medium are sterilised by autoclaving or supplied pre-sterilised. The plate is inoculated using sterile loops, swabs or pipettes and incubated upside down to prevent condensation from dripping onto the surface. Sterile technique and clean working environments reduce contamination and ensure that resulting colonies arise from the inoculated sample rather than airborne organisms. Variants of the basic design include segmented dishes for multiple samples and specialised plates with vents to control moisture.
Practical Applications in Microbiology
Petri dishes are widely used tools in microbiology and related disciplines. Streak plate and spread plate methods use them to isolate single colonies from mixed samples, allowing researchers to establish pure cultures for identification and further study. Quantitative pour plate techniques measure colony-forming units to estimate the number of viable microorganisms in water, food or clinical specimens. Antibiotic susceptibility testing, such as the Kirby-Bauer disc diffusion assay, places antibiotic-impregnated discs on inoculated agar to measure zones of inhibition. Environmental swabs from surfaces or air samples are cultured on Petri dishes to monitor contamination in hospitals, food production and cleanrooms. Mycologists culture molds and yeasts to study their growth patterns and to screen for antifungal compounds. Plant scientists use agar plates to germinate seeds or maintain small plant tissues under sterile conditions. The visual appearance of colonies, including their colour, shape, size and texture, provides clues to the identity and physiology of the organism.
In summary, the Petri dish remains a simple but widely used laboratory vessel that supports the growth and study of microorganisms and small tissues. Its standardized design, adaptability and compatibility with sterile technique make it a cornerstone of microbiological research, diagnostics and teaching.
Related Terms: Agar medium, Culture plate, Microbial culture, Colony, Inoculation loop